A level shifter generally functions for shifting a voltage level in circuitry. FIG. 1 illustrates a conventional level shifter 10, which includes an input buffer 102, an output buffer 106, and a level-processing unit 104. The input buffer 102 is biased between a first voltage source VDD and ground voltage GND. The level-processing unit 104 is coupled between the input buffer 102 and output buffer 106, and biased between a second voltage source VPP and ground voltage GND, wherein VPP is higher than VDD. The output buffer 106 coupled to the level-processing unit 104 is also biased between VDD and GND. Through the level shifter, an input signal Vin with a voltage level ranged between VDD and GND can be shifted to an output signal Vout having a voltage level ranged between VPP and GND, thereby achieving the object of level shifting.
In the above-mentioned circuitry, since the first voltage source VDD and the second voltage source VPP are independent from each other, the synchronous supply of the two voltage sources cannot be guaranteed. In a case that the second voltage source VPP is switched on while the first voltage source VDD still stays off, the input buffer 102 requiring VDD is unable to work normally. Thus the voltage at its output end is unknown and the output voltage of the output signal Vout will be consequently abnormal. As a result, the operation of the entire system would be adversely affected.